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The total non-story of the Fukushima nuclear powerplant "disaster" – which has seen and will see no deaths or measurable health consequences for anyone anywhere – has received a shot in the arm today with the news that Japanese authorities have upgraded the incident to a Level 7 on the nuclear accident scale.

"Radiation in Japan is as bad as Chernobyl ... level is raised to 7 for only the second time in history ... spread of radioactive particles is out of control ... Lasting horror: Ukrainian children suffering from cancer caused by radiation from the Chernobyl disaster"

The facts are that the incident at Fukushima Daiichi remains far and away the most minor of the various consequences which have followed the initial, devastating magnitude-9.0 quake and tsunami which struck northeastern Japan nearly a month ago. It has caused less human consequences than a moderate road-traffic accident. The nuclear reactors in the stricken provinces came through mostly unscathed (even at the Daiichi site two are expected to return to service, and at other nuclear powerplants in the region no significant damage at all was seen). One nuclear worker, in a crane cab at the time, was killed by the quake strike at the Daini plant: two were killed by the tsunami wave at Daiichi. A handful have been injured by the quake and following hydrogen explosions.

Almost all other infrastructure hit by the natural disaster failed catastrophically. Housing, transport and industry across the region collapsed with deadly consequences, killing people by the tens of thousands. Oil plants, chemical factories, storage facilities and tankers of every type ruptured and burned, spilling megatonnes of pollution and carcinogens into the environment. But almost nothing is heard of all this, except as a footnote to the supposed radiological hazards resulting from the Fukushima Daiichi reactors 1 to 4.

So what's happening in and around the Daiichi plant?

Residual heating in the cores at reactors 1 to 3 has now decayed down to less than 0.37 per cent of normal output power. It is this heating which has previously driven emissions of core material from the cores, and which plant personnel struggled to control in the hours and days after the tsunami knocked out backup cooling power and backup-backup batteries were exhausted. During that time heating levels, though falling fast, were initially 20 times what they are now.

Steam vented from the hot cores was and is not dangerous in itself, but some material from the fuel rods themselves was naturally carried out along with vented steam. Some of this was carried off by the wind to be deposited in the area around, where it is detectable in minuscule amounts.

Most media have chosen to report Japanese government calculations indicating (three-page PDF/56.7 KB) that perhaps 10,000 terabecquerels per hour of iodine-131 may have been emitted from the Daiichi cores in the hours following the initial decision to vent them. This is assessed as around 10 per cent of the emission levels seen at Chernobyl.

That's largely meaningless, however. If all the iodine emitted in one hour had been sitting still at a single point (no) and that had been the only radio-isotope present (no again) you could have stood 100 metres from that point for three hours and suffered zero health consequences. Becquerels of a given isotope don't relate closely or directly to health consequences: we need to look at dose rates instead.

At times, close to reactor buildings on the Daiichi site, radiation dose rates as high as 1,000 millisievert/hour have been recorded by remote instruments. That is serious radiation: after an hour exposed to it you'd be likely to suffer actual radiation sickness, though you'd be just about certain to recover. Two hours, and you might die: four hours, a fatal result would become likely. If millions of people were exposed to such levels for say a quarter of an hour, decades later you'd be able to point to increased cancer rates among them (though the risk to any individual would be negligible).

But these were in fact very brief spikes right next to a damaged core, resulting mostly from very short-lived isotopes that were decaying before they could drift beyond the plant fence. Nobody at all has been exposed to such levels.

Thus far the worst exposure was suffered by three workers who stood in ankle-deep radioactive water for several hours and sustained doses above 100 millisievert from doing so, indicating local levels of 20-odd millisievert/hour. They have suffered zero health consequences as a result. As of the latest reports, as many as four other workers (of all the many hundreds present at the site) have gone above 100 millisievert: the maximum level allowed is 250 before being withdrawn from the operation altogether, but as is common in the nuclear industry intense caution is being exercised.

Danger beyond the plant fence has remained effectively nil. As of yesterday, according to nuclear experts at MIT in the States (reviewing data from Japanese and international monitoring teams on the ground) the highest dose rates seen within 30km of the plant have been 0.0016 millisievert/hour.

For context, you could live permanently under radiation levels of 0.0016 mS/hr and you would never achieve even half the annual dose levels permitted by airline crew.

The only actual health menace of any kind beyond the plant fence from Fukushima (and indeed following Chernobyl) has been presented by ingestion of radioisotopes in food: specifically of radioisotopic iodine. For adults this appears to have almost no effect, but in the case of children radio-iodine is taken up and concentrated in the thyroid gland very efficiently. Even though it decays away completely in a matter of weeks (iodine-131 has a half-life of just eight days), if a child ingests even quite small amounts of radio-iodine he or she will have a tiny extra risk of thyroid cancer in future – about 0.02 per cent, based on Chernobyl.

Fortunately, thyroid cancer – unusually among cancers – is almost always curable without ill effects (this is done, counterintuitively, using much larger amounts of iodine-131) and so the chance of such a child actually dying as a result of such exposure is unfeasibly tiny: less than one chance in a million.